1. Field of the Invention
The invention relates generally to television receivers, which are also called display monitors, and more particularly to integrated circuits designed for the control of image displays on a cathode-ray tube.
2. Discussion of the Related Art
Image display screens based on tubes use a mode of scanning the screen by means of an electron beam. The beam scans the screen line by line and successively strikes all the luminophors deposited on the surface of the screen. The electron beam is modulated in intensity during this scan to form light dots of variable intensity on the screen. The combination of these dots form the desired image.
A control circuit, hereinafter called a screen controller, is an integrated circuit used to control the electronic circuits that set up the scanning by the beam, as well as to control the electronic circuits that modulate the intensity of the beam during this scanning operation.
Deflection of the beam along a horizontal line is produced by a horizontal deflection coil that is crossed by a periodic variable current, which produces a periodic variable magnetic field. The magnetic field deflects the beam along an image line. At the end of the line, the current in the coil is abruptly canceled so that the beam swiftly returns to the beginning of a line. This sudden cancellation produces a so-called "line flyback" pulse used as a negative feedback signal to synchronize the scanning of the screen with the video image signal.
During this time, a vertical deflection coil receives a periodically variable current, with a far longer duration than that of the variable current for the horizontal deflection coil, so that the beam shifts gradually in the vertical direction and starts on another line after the end of a previous line. At the end of a frame, the current in the vertical deflection coil is suddenly canceled and the spot swiftly returns to the start of the first line. A frame flyback pulse is produced at this time under the effect of the abrupt cancellation of current.
In practice, the composite video image signal 1 used by the screen controller can be subdivided into several parts for each image line as shown in FIG. 1a. A first part includes a line synchronization square-wave pulse, HSYNC, which is typically negative. A second part is a signal at zero enabling the black level to be adjusted. A third part is the useful signal for the modulation of the intensity of the beam which is the positive signal. The horizontal deflection current 2 is applied, as shown in FIG. 1b, for the duration of the useful signal. It is a linearly rising current that returns to zero at the end of the useful part of the signal. It is held at zero for a period of time approximately equal to about 12 microseconds, starting slightly before the beginning and ending slightly after the end of the line synchronization square-wave pulse. This interruption of the horizontal deflection current is therefore synchronized by the line synchronization square-wave pulse of the composite video signal.
The line 3 of FIG. 1c represents the line flyback pulse put out by the horizontal scanning circuit during the cancellation of the horizontal deflection current.
Periodically, the composite video signal also comprises a frame synchronization square-wave pulse 4, as illustrated in FIG. 1d. The frame synchronization square-wave pulse 4 is distinguished by its longer duration from the line synchronization square-wave pulse. Only the frame synchronization square-wave pulse is shown in FIG. 1d. A vertical deflection current 5, illustrated in FIG. 1e, is produced between two successive frame synchronization square-wave pulses. This vertical deflection current 5 is periodically interrupted to enable the return of the spot to the start of the frame and this interruption is triggered by the frame synchronization square-wave pulse 4.
The line 6 of FIG. 1f represents the frame flyback pulse produced by the vertical scanning circuit at the end of a frame.
During the time interval corresponding to the return of a spot to the start of a line and during the time interval corresponding to the return of a spot to the start of a frame, the electron beam is extinguished or "blanked" so as not to cause the appearance of luminous flyback trails on the screen. This blanking of the beam is produced by the application of a sufficiently negative voltage to a grid placed before the cathode of the electron gun of the tube or by the application of a sufficiently positive voltage to the cathode.
In the prior art, the blanking voltage is applied in two instances.
The blanking voltage is applied first at each end of a line for a duration of some microseconds needed for the return of the spot to the beginning of the line and for the reactivation of the horizontal deflection current. The blanking is produced then during a temporal square-wave pulse defined by a command signal HFBACK that is generated from the line flyback pulse and synchronized with the synchronization square-wave pulse present in the composite video signal.
The blanking voltage is also applied at each end of a frame, during a frame blanking square-wave pulse (1 to 2 milliseconds) corresponding to the frame flyback pulse, as well as during a frame synchronization square-wave pulse present in the composite video signal (about 0.5 milliseconds). The blanking is then produced firstly during a temporal square-wave pulse corresponding to the frame synchronization signal VSYNC and secondly during a temporal square-wave pulse defined by a control signal VFBACK which is generated from the frame flyback pulse.
Consequently, a blanking command logic signal called a blanking signal BLK is generated from a logic OR function receiving the signal HFBACK, the signal VFBACK and the signal VSYNC.